The present invention is directed to a system and method for stabilizing the frequency of the optical output signal of a laser, and more specifically for stabilizing the output signal by applying a corrective signal to the output signal without modification to the internal laser cavity.
The frequency stability of lasers allows for their application in interferometry. In interferometry, the laser beam is split into two paths and then recombined. A difference in the optical path length between the two paths produces a differential phase shift between the two recombined beams. This phase shift modulates the intensity of the recombined beams where the maximum intensity is achieved when both beams are in phase, and minimum intensity occurs when the two beams are 180° out of phase. Information from the interferometer requires accurate measurement of this intensity to determine the differential phase. If the laser output is not stable in frequency, when the beams from two unequal paths are recombined, the frequency jitter of the laser will produce differential phase jitter in the recombined beams. The amplitude of the phase jitter increases with increasing frequency jitter and increasing difference in the optical path lengths of the two beams The frequency stability of a laser can be adversely affected by thermal effects, vibration, and other influences that are difficult and expensive to reduce or completely eliminate.